Anatomy of a Class: Practice Questions (Computer Science A 3.3)

These are original Marlbridge practice questions, not past exam questions. All scenarios are fictional. Mark points are a suggested Marlbridge rubric, not an official scoring guideline. No calculator is allowed. Assume code fragments compile unless a question says otherwise. The Parcel class used in several questions is the one in the study guide.

Question 1 (multiple choice · foundation)

In this course, which declaration pattern is expected for a class with ordinary instance variables, a constructor and methods?

• (A) A private class, public instance variables, private constructors
• (B) A public class, private instance variables, public constructors
• (C) A public class, public instance variables, private constructors
• (D) A private class, private instance variables, public constructors

Question 2 (multiple choice · foundation)

What does the keyword private do when it appears before an instance variable?

• (A) It prevents the variable from ever changing.
• (B) It restricts access to code inside the declaring class.
• (C) It makes one shared copy of the variable for all objects.
• (D) It allows only the constructor to read the variable.

Question 3 (multiple choice · core)

A class Parcel declares private double weightKg;. Another class contains this code.

Parcel p = new Parcel(3.0, "Bath");
p.weightKg = 30.0;

What happens?

• (A) The program compiles and sets the weight to 30.0.
• (B) The program compiles, but throws an exception when it runs.
• (C) The program does not compile, because weightKg has private access in Parcel.
• (D) The program compiles and ignores the assignment.

Question 4 (multiple choice · core)

Consider the Parcel class from the study guide and this code segment.

Parcel a = new Parcel(25.5, "Leeds");
Parcel b = new Parcel(3.0, "Bath");
a.markDelivered();
System.out.println(a.describe() + " " + b.isHeavy());

What is printed?

• (A) Leeds (on the way) false
• (B) Leeds (delivered) true
• (C) Leeds (delivered) false
• (D) Bath (on the way) false

Question 5 (multiple choice · core)

A Scoreboard object sb has just been constructed (both scores 0). Its addHome and addAway methods ignore any argument that is not positive, and leader returns "home", "away" or "tied".

sb.addHome(3);
sb.addAway(5);
sb.addAway(-2);
sb.addHome(2);
System.out.println(sb.leader());

What is printed?

• (A) home
• (B) away
• (C) tied
• (D) Nothing: the program does not compile.

Question 6 (multiple choice · core)

A class Scoreboard has a method private boolean isValid(int points). Which statement is true?

• (A) Any class may call isValid through a Scoreboard object.
• (B) Only methods and constructors inside Scoreboard may call isValid.
• (C) isValid can only be called by the constructor.
• (D) isValid cannot be called at all.

Question 7 (constructed response · core)

Write a complete class LibraryCard from this specification.

• It stores the owner’s name (a String) and the number of books currently out (an int).
• The constructor takes the owner’s name and starts with 0 books out.
• borrow() allows a book only if fewer than 5 are out. If allowed, it adds one and returns true; otherwise it returns false.
• getBooksOut() returns the number of books out.

public class LibraryCard
{
    private String ownerName;
    private int booksOut;

    public LibraryCard(String name)
    {
        ownerName = name;
        booksOut = 0;
    }

    public boolean borrow()
    {
        if (booksOut < 5)
        {
            booksOut++;
            return true;
        }
        return false;
    }

    public int getBooksOut()
    {
        return booksOut;
    }
}

Question 8 (constructed response · core)

A student wrote this class.

public class Cup
{
    public int millilitres;
    private String colour;

    public void Cup(int ml, String c)
    {
        millilitres = ml;
        colour = c;
    }

    public String getColour()
    {
        return colour;
    }
}

(a) A different class contains Cup c = new Cup(250, "red");. Explain why this does not compile.

(b) Identify a second problem that breaks the principle of encapsulation.

(c) Rewrite the class so that both problems are fixed.

public class Cup
{
    private int millilitres;
    private String colour;

    public Cup(int ml, String c)
    {
        millilitres = ml;
        colour = c;
    }

    public String getColour()
    {
        return colour;
    }
}

Question 9 (constructed response · stretch)

A developer wants the weight of a Parcel never to be negative.

(a) Explain why making weightKg private helps.

(b) Write a public method setWeightKg(double w) for Parcel that changes the weight only if w is positive.

(c) Explain how this shows encapsulation.

public void setWeightKg(double w)
{
    if (w > 0)
    {
        weightKg = w;
    }
}

How did you do?

• Questions 1 to 6 right and the rubrics mostly met: move on to the Constructors study guide.
• Missed Questions 1, 2 or 6: reread “Access: public and private” in the study guide.
• Missed Questions 3, 4 or 5: redo Worked example 1 and “What the compiler enforces”.
• Missed the writing questions: redo Worked example 2, then tick the topic checklist.